Rotors for use in motors such as brushless DC motors include permanent magnets held within the rotor. Torque is applied by a rotating magnetic field. This is created by applying current, usually in three separate phases, to stationary coils mounted in an outer stator. This type of motor is becoming increasingly popular due to advances in the control and power electronics required to electrically commutate the current supply to the stator. Such machines are often called motors but may equally be used as motors, generators or motor/generators.
In existing rotor configurations, the permanent magnets are retained in position by a cylindrical sleeve which extends around the outer circumferential surface of the rotor.